The subject invention relates to a method of removing an acid gas such as carbon dioxide from a vapor or a flowing gas stream. The subject invention is specifically directed to the adsorptive separation of carbon dioxide from air using a carbon molecular sieve-based material which has been functionalized with amine groups.
Porous carbon-based materials have found a broad range of uses as adsorbents for treating various materials and as adsorbents employed in separation processes. This utility has led to a significant amount of activity in developing methods to produce better carbon molecular sieves as represented by the references cited below and the material referred to in these references.
U.S. Pat. Nos. 4,540,678; 4,627,857; and 4,629,476 issued to R. F. Sutt, Jr. are believed relevant for their general teaching in regard to the preparation of carbon molecular sieves. These references describe the preparation of carbon molecular sieves from carbon sources such as coal or coconut char. The references also indicate that it is known to produce the sieves by treating the feed material in a substantially oxygen-free environment during a heating or calcining step. According to these references, the pore distribution may be adjusted through the use of a pore blocking substance such as a large molecular weight hydrocarbon although this material is not employed in all embodiments described in these three references.
U.S. Pat. Nos. 4,046,709 issued to N. Yuki and 4,420,415 issued to N. Yuki et al. are believed relevant as they describe the production of carbon molecular sieves from polymeric precursor materials similar to that which may be employed in the subject invention. These references describe the utilization of a vinylidene chloride copolymer as a precursor material. The copolymer is carbonized for the removal of its hydrogen and chlorine components and the resultant char is pulverized. Sintering agents such as coal tar pitch and binder materials such as cellulose are then added to pulverized product and the mixture is shaped. The thus shaped bound material is then subjected to a high temperature carbonizing treatment to yield the finished material.
U.S. Pat. No. 3,960,768 issued to W. Ripperger et al. is also believed relevant for its teaching in regard to the production of carbon molecular sieves from polymerized halohydrocarbons by the elimination of hydrogen halide during a heating procedure. The reference indicates appropriately shaped end products can be obtained by using raw materials having a corresponding shape. The teaching at column 3 of this reference indicates the preshaped starting materials shrink into the corresponding shapes.
The last cited reference teaches a specific method of heating the polymer in order to avoid the problem associated with the prior methods. More specifically, in the prior methods such as that of U.S. Pat. No. 4,420,415 cited above the polymer would tend to produce a foam-like highly porous carbon upon heating and removal of hydrogen chloride. The resultant material was too soft and porous to be utilized effectively in commercial applications. For this reason, it is necessary to grind the foam-like material to a powder and employ a binder to form articles having a desired finished shape.
There have been references in the literature to carbon molecular sieves which have a hydrophobic nature. For instance, this is described in the article at page 512 of Volume 271 of Nature (1978) by E. M. Flannigen et al.
It is known in the art to chemically treat the surface of a carbon-based material to improve its adsorptive capabilities. For instance, U.S. Pat. No. 3,491,031 issued to G. R. Stoneburner describes the preparation and regeneration of activated carbons which are impregnated with monoethanol amine (MEA). These carbons are described as being effective in removing carbon dioxide and other acid gases from ethylene, nitrogen or air. This reference also presents a regeneration method which comprises passing an additional amount of amine vapor across the carbon to prevent amine losses or decomposition during regeneration.
Other references in which an amine is employed to enhance the adsorptive properties of a carbon-based support are U.S. Pat. Nos. 4,320,011 and 4,531,953.